Dentists use dental impression trays and cups to make impressions of teeth. Dental impressions should essentially be an exact replica, in reverse, of the existing dentition, the surrounding tissues and support structures. The impression is used to produce a gypsum model upon which a prosthetic or corrective device can be fabricated. If there are distortions or inaccuracies in the dental impression, the resultant model will be inaccurate and therefore the fabricated appliance will not fit correctly creating pain, trauma and difficulty for the patient.
Generally, dental trays, both open mouth and closed mouth, past and present, appear to have two (2) features that prevail:
1. The trays have dental impression material containment sidewalls.
2. The trays provide a tray floor or a mid-section separator sheet to section the tray into an upper and lower chamber to contain the dental impression material.
U.S. Pat. No. 310,407 discloses dental impression trays that are generally shaped to conform to a dental arch and include a trough type tray surrounded by solid impression material containment walls. The tray serves as a carrier for the impression material, it additionally serves to hold the positioned material in place through the setting process. This tray is used for open mouth dental impressions. U.S. Pat. No. 1,360,146 discloses a dental tray which may be used to capture an upper and lower partial impression and to capture the bite in one operation. This tray is U shaped and consists of opposing buccal and lingual solid sidewalls. The use of this tray is described as being suitable for closed bite partial impressions and the practical operation for use is described as mounting an impression material between the sidewalls on opposite sides of a narrow piece of a rubber dam. Effectively, this description teaches that the rubber dam, would function as a mid-section to separate the tray into an upper and lower chamber. The illustration does not show the rubber dam.
It appears that the dental tray art seeks to reduce distortion of the impression and inaccuracies in the bite registration. Distortion can occur due to biting forces. The biting forces cause the impression material to extrude in all directions placing pressure on the sidewalls of the tray causing the sidewalls to flex and remain flexed through the setting process. The walls then relax back and bring with them, to a degree the set material. Distortion of the impression can also occur due to the tray separating from the material in the process of trying to remove it from the mouth. Distortion of the impression can further occur due to the manipulation of the tray during the model preparation phase.
In addition to distortion, inaccuracies of the bite registration can occur. Inaccuracies occur due to the thickness of a separator sheet that divides the upper and lower chamber preventing the teeth from making full surface-to-surface contact.
There have been several attempts to address distortions and inaccuracies. Traditionally, it appears the attempts have followed in the direction of:
a. stronger, reinforced sidewalls, to prevent flexing;
b. perforated sidewalls to retain impression material in the tray and keep it from separating during removal from the mouth; and
c. thinner separating sheets made of various preferred materials.
Examples of closed bite dental trays seeking to reduce distortion and inaccuracies are shown below in the following described patents.
U.S. Pat. Nos. 1,584,092 and 1,979,493 disclose and describe closed mouth impression trays having solid sidewalls to contain the impression material and a separator sheet that sections the tray into upper and lower chambers.
U.S. Pat. Nos. 2,313,535 and 2,597,929 disclose and describe a wire form that conforms only to the dental arch buccally. The position of metal retention clips mid-way in the central bite area would prevent the patient with posterior teeth from closing into full surface to surface, centric occlusion and render these devices as unsuitable for use as a closed bite dental impression tray.
U.S. Pat. No. 2,703,452 discloses a tray with perforated sidewalls and a separator sheet.
U.S. Pat. No. 3,468,029 discloses a closed bite impression tray that consists of two matching trays with half height sidewalls that are connected by a posterior hinge that allows the tray to be opened for insertion of a separator sheet that effectively sections the ready tray into an upper and lower chamber and secondarily allows for the tension of the separator sheet to be adjusted.
U.S. Pat. No. 3,604,116 discusses the inaccuracies of bite registrations obtained in closed bite impression trays with a separator sheet having a thickness greater than. 0.020 mm. U.S. Pat. No. 4,003,132 discloses a closed bite dental impression tray that consists of perforated sidewalls and a separator sheet that sections the tray into upper and lower material containment chambers.
U.S. Pat. No. 4,204,323 discloses a closed bite dental impression tray with perforated side walls that additionally contain an interior locking recess and a slot for placement of a separator sheet to section the tray into upper and lower material containment chambers.
U.S. Pat. No. 4,368,040 discloses a dental impression tray with a material chamber tray floor. Impression material may be injected into the tray through slotted sidewalls to obtain a dental impression insitu.
U.S. Pat. Nos. 4,449,927 and 5,820,372 disclose a two part system consisting of a bite tray holder with a pair of laterally spaced, solid side wall tray holders shaped to receive an insert-able bite tray and the insert-able bite tray with solid side walls and a separator sheet that sections the tray into upper and lower material containment chambers.
U.S. Pat. Nos. 4,619,610, 4,689,010, and 5,316,474, disclose trays with solid sidewalls with interior grip mechanisms and a separator sheet.
U.S. Pat. No. 5,636,985 discusses in length the distortion problems and discloses a tray with extra-ridged solid sidewalls and a separator sheet.
Impression trays used for open mouth impressions have followed a similar course as evidenced by U.S. Pat. Nos. 3,473,225, 5,336,086, and 6,079,977. In addition to trays used for closed bite impressions of the upper arch, lower arch and the bite registration, there are secondary dental impression trays that are used primarily to capture only the bite registration. The same problems of bite inaccuracies due to a separator sheet are prevalent. For example, U.S. Pat. Nos. 2,713,202, 2,895,219 and 3,903,602 disclose a pair of frames connected by a handle and have contained thereon, bibs for the placement of impression material. This patent does not discuss the use of the frames without the bibs. The frames do not extend vertically.
U.S. Pat. No. 4,472,140, discloses a plastic frame that partially surrounds a dental arch and contains a thin separator sheet for the placement of impression material. The frame does not extend vertically.
Finally, commercially available is an open walled dental tray fabricated of multiple 4–6 gage welded stainless steel rods. It is designed and intended to be re-useable from one patient to another by means of cleaning and sterilization.
Problems concerning distortion, retention and inaccuracies continue to exist.